Previously the commercial practice in forming a fused multifocal lens blank consisted in separately forming the major elements of the blank by grinding and polishing a circular countersink sufficiently large to receive a ground and polished minor element which had been ground and polished on one surface to the contour approximating that of the countersink in the major element. The major and minor polished surfaces opposing each other were combined and heat joined at a fusion temperature compatible with the materials from which the major and minor elements were composed. Problems of air entrapment were experienced in the foregoing procedure resulting in imperfect blanks, and this procedure was time-consuming and costly.
In some present multifocal lens blanks, the perimeter of the segment of the countersink is not always circular and may take the configuration of a semicircle, rectangle or other geometric form. A broad range of geometric shapes makes it difficult, if not economically feasible, to form correspondingly shaped countersinks in the major lens blank by grinding and polishing, and the equally difficult task is experienced with respect to the formation of a cooperating minor element for introduction and cooperation with the specific irregular geometric countersink in the recipient major lens blank. Assembling the polished surfaces of the major and minor blanks opposing each other and then fusing them together has also encountered substantial difficulties and problems.
Another method employed in the formation of a non-circular minor element in a multifocal lens blank is to heat-join the two flat edges of a semicircular-shaped crown glass carrier element and a semicircular-shaped barium glass minor element, and then grind and polish one surface of the assembly to a contour closely approximating that of the circular countersink in the major lens blank. The two polished surfaces are then placed together opposing each other and when fused together, only the minor segment is visible. The foregoing methods of forming heat-joined multifocal lens blanks not only require additional glass pressings but also necessitate extra fusing operations including careful cleansing of the surfaces of the several components to eliminate air entrapment and interfacial bubbles that may be entrapped between the surfaces being fused. The mating surfaces require special treatment in order to avoid liberation of gases during fusion to insure that no impurities or any air entrapment would cause the liberation of gases. Problems with alignment with respect to the major and minor elements of the multifocal lens are ever present during the fusing procedure while continuously endeavoring to avoid the entrapment of air bubbles between the elements.
An initial attempt to overcome many of the problems in the method of forming fused bifocal lens blanks is disclosed in my patent, U.S. Pat. No. 2,734,315, which was assigned to Pittsburgh Plate Glass Company, in which fusion of a minor element of the lens to the major element occurred by depositing molten glass of a suitable composition upon the polished surface of the countersink in the major element without entrapment of air at the fusion interfaces. However, the countersink surfaces were ground and polished surfaces as opposed to the present utilization of pressed and unpolished countersinks in the major lens blanks which countersinks have a broad variation of geometrical configurations that do not lend themselves to grinding and polishing of prior practices.
Therefore, it is an object of the present invention to provide a method of forming a fused multifocal lens blank by first pressing a suitable countersink of the desired geometrical configuration in the major lens blank when it is being formed, and preferably then transferring the heated blank with the pressed countersink while mounted on a suitable lens blank support and maintained at an elevated temperature for positioning of the heated lens blank having the present countersink beneath the discharge orifice of a molten glass dispensing unit to receive into the pressed countersink molten glass of a suitable refractive index to be fused with the lens blank pressed countersink after which the fused composite multifocal lens blank may be subjected to a cooling cycle before annealing to relieve any stresses induced during the fabrication.
Other objects and many of the attendant advantages of this invention will become more readily apparent to those skilled in this art of lens production from the accompanying drawing and detailed description which follows, with the understanding that the claims appended are not to be limited and equivalents are contemplated.